The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known from the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
The evolution of communication technology, particularly IP-based (IP, Internet Protocol) communication technology and end user terminals, has enabled versatile communication possibilities and introduction of different services. More and more often services are implemented using an IP Multimedia Subsystem IMS, such as an All-IP system standardized by the 3GPP, and employing an IP-based session control protocol, such as a SIP (Session Initiation Protocol) protocol. IMS provides multimedia services which are usually, although not necessarily, Internet-based services employing a packet protocol. SIP is an IETF defined application-layer control protocol which is not vertically integrated into a communications system but a tool to build a multimedia architecture.
In order to ensure interworking between IMS networks and an exist circuit-switched network infrastructure, such as GSM (Global system for mobile communications), a platform called an MSC (mobile switching centre) server system has been developed. The MSC server system is independent of the transmission backbone networks, and it separates call control and signalling (control plane) and traffic (user plane) into separate network nodes, so that a network node called an MSC server (MSC-S) is responsible for call control, interworking with other domains and signalling, and a network node called a multimedia gateway (MGW) is responsible for switching and carrying the actual traffic. Although the separation enables the optimization of the traffic flows by locating MGWs closer to end users and by locating MSC-Ss near interconnection points, the traffic and signalling to or from a roaming subscriber still has to be circulated via a home network of the roaming subscriber in order to keep the connection management in the home network.